Better implementation for invalid?

[istathar]

A long while ago we added invalid in Core.Program to make it clear when you'd hit an illegal state (when in monadic context).

unbeliever/core-program/lib/Core/Program/Execute.hs

Lines 1103 to 1108 in e55e65f

	{- |
	Illegal internal state resulting from what should be unreachable code or
	otherwise a programmer error.
	-}
	invalid :: Program τ α
	invalid = error "Invalid State"

is the current implementation

I bet that could be done better.

At the very least an HasCallStack => constraint might be good, but is there something better than using error here? MonadFail fail?